Deck ledger board: size, fasteners and flashing

What the ledger does and why it fails

The ledger is a pressure-treated board bolted to the house that carries one end of every joist. When it fails, the deck does not sag — it drops, all at once, usually with people on it. Investigations of residential deck collapses point overwhelmingly at this one connection.

The two failure modes are simple. Either the fasteners were never adequate — nails, deck screws, or lag screws at too wide a spacing — or water got behind the board and rotted the house's band joist, so there was nothing left for the fasteners to hold. Both are entirely preventable and both are invisible once the decking is on.

Ledger size and what it can be fixed to

The ledger matches the joists it carries: 2x8 joists get a 2x8 ledger, 2x10 joists a 2x10. It must be pressure-treated and rated for the exposure. It has to bear on the house's rim or band joist, or on solid blocking installed for the purpose.

It cannot be fixed to any of the following, and this is where homeowner-built decks most often go wrong:

• Brick or stone veneer. Veneer is a skin, not structure — it carries its own weight and nothing else.
• Stucco, EIFS or foam sheathing over the framing. Fasteners through more than half an inch of sheathing lose their rating, and compressible foam is worse still.
• A cantilevered floor, such as a bump-out or bay. The floor joists there are already working at their limit.
• Hollow masonry block without grouted cells and proper anchors.
• Siding. A ledger must sit against solid sheathing or the band joist with the siding cut back, never over the cladding.

If the wall is one of these, the deck goes freestanding on its own posts and beams. That costs an extra beam and another row of footings and is often the easier inspection anyway — see freestanding versus attached.

Fastener spacing comes from a table, not from judgement

The American Wood Council's DCA 6 prescriptive guide sets ledger fastener spacing directly from the joist span. Longer joists put more load into the connection, so the fasteners close up. These are the values for a Southern Pine, Douglas Fir-Larch or Hem-Fir ledger against a 2-inch nominal solid-sawn band joist, in inches on centre:

Through-bolts allow much wider spacing than lags because they clamp the whole assembly rather than relying on thread withdrawal — but they need access to the band joist from inside, which a finished basement rarely gives you. The ledger board calculator turns your span and ledger length into a fastener count.

Layout rules

• Two staggered rows, one in the top third of the board and one in the bottom third.
• At least 2 inches from the top and bottom edges of the ledger.
• At least 2 inches and no more than 5 inches from each end of the board.
• At least 1 5/8 inches apart vertically and 3 inches apart horizontally between the two rows.
• Never fewer than two fasteners, even on a very short ledger.
• Pilot-drill lag screws. A lag driven without a pilot hole splits the band joist it is supposed to be holding.

Many jurisdictions now also require lateral load connectors — hold-down tension devices tying the deck framing back into the house's floor system at two or four points. They resist the deck being pulled away from the wall, which lag screws in shear do not do well. Ask your inspector whether yours are required.

Flashing: the order matters more than the material

Water running down the wall has to get past the ledger without getting behind it. That is achieved by lapping, not by sealant.

1. Cut the siding back so the ledger sits against the sheathing or band joist, not on the cladding.
2. Tuck the house wrap or building paper so it laps over the top flange of the flashing, never under it.
3. Set the flashing behind the ledger with a leg that turns down over the face of the board.
4. Fasten the ledger through the flashing, then seal the fastener heads, not the joint.
5. Fit the siding back down over the flashing's upper leg with a gap so water can drain out.

Use copper, stainless or a vinyl flashing. Aluminium corrodes on contact with the copper-based preservatives in modern pressure-treated lumber, and the corrosion happens exactly where nobody can see it. Some jurisdictions inspect the flashing separately before the decking goes on; where they do, do not cover it up first.

Checking an existing ledger

On a deck you did not build, four checks tell you most of what you need to know. Look for flashing above the ledger — if there is none visible, assume water has been getting behind it. Look at the fastener heads: hex heads in two staggered rows are a good sign, a row of screw heads or nail heads is not. Push a screwdriver into the ledger and the siding just above it and see whether the wood gives. And look at what the ledger is fastened to, because a ledger on brick veneer is a problem no amount of extra bolts will solve.

If any of those raise a doubt, the fix is usually to convert the deck to freestanding rather than to add fasteners into rotten wood.